1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly to an organic light emitting display capable of displaying an image of uniform luminance regardless of a leakage current.
2. Discussion of Related Art
Various flat plate displays with reduced weight and volume in comparison to cathode ray tubes (CRT) have been developed. Flat panel displays include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and organic light emitting displays.
Among the flat panel displays, the organic light emitting displays make use of organic light emitting diodes that emit light by re-combination of electrons and holes. The organic light emitting display has advantages of high response speed and small power consumption.
FIG. 1 is a circuitry diagram showing a pixel of a conventional organic light emitting display.
With reference to FIG. 1, the pixel 4 of a conventional organic light emitting display includes an organic light emitting diode OLED and a pixel circuit 2. The pixel circuit 2 is coupled between a data line Dm and a scan line Sn, and controls the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled with the pixel circuit 2, and a cathode electrode thereof is coupled to a second power supply ELVSS. The organic light emitting diode OLED generates light of a predetermined luminance corresponding to an electric current supplied from the pixel circuit 2.
When a scan signal is provided to a scan line Sn, the pixel circuit 2 receives a data signal from the data line Dm, and controls an amount of an electric current supplied to the organic light emitting diode OLED according to the data signal. In order to do this, the pixel circuit 2 includes a first transistor M1, a second transistor M2, and a storage capacitor Cst. The second transistor M2 is coupled between a first power supply ELVDD and the organic light emitting diode OLED. The first transistor Ml is coupled among the second transistor M2, the data line Dm, and the scan line Sn. The storage capacitor Cst is coupled between a gate electrode and a first electrode of the first transistor M1.
A gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode thereof is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled with the gate electrode of the second transistor M2. The first and second electrodes may be either the drain or source electrodes. For example, the first electrode may be the source electrode and the second electrode may be the drain electrode. When a scan signal is supplied to the scan line Sn, the first transistor M1 is turned on to supply the data signal from the data line Dm to the gate electrode of the second transistor M2. At this time, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and a first electrode thereof is coupled to another terminal of the storage capacitor Cst and the first power supply ELVDD. The second electrode of the second transistor M2 is coupled to an anode electrode of the organic light emitting diode OLED. The second transistor M2 controls an amount of an electric current from the first power supply ELVDD to the second power supply ELVSS through the organic light emitting diode OLED according to the voltage stored in the storage capacitor Cst. The organic light emitting diode OLED generates light corresponding to an amount of an electric current supplied from the second transistor M2.
So as to express a desired image in the aforementioned pixel 4, the voltage charged in the storage capacitor Cst, namely, the voltage corresponding to the data signal should stably maintain during one frame. However, in the conventional pixel 4, during a displayed time period of the image, a predetermined leakage current from the storage capacitor Cst is supplied to the data line Dm through the first transistor M1.
As described earlier, when the leakage current occurs, the voltage stored in the storage capacitor Cst varies. According to a variation voltage of the storage capacitor Cst, when the luminance of the red pixel R, the green pixel G, and the blue pixel B change by the same value, a uniform image may be displayed. Accordingly, observers cannot recognize the change in luminance due to the leakage current.
However, due to properties of materials of red, green, and blue light emitting diodes included in the red, green, and blue pixels, although voltage variation amounts of the storage capacitor Cst are identical with each, an amount of light generated by the red, green, and blue pixels may be different from each other.
That is, emission efficiency may be expressed by the following equation 1 based on properties of current used materials:OLED(G)>OLED(R)>OLED(B)  (1).
Accordingly, when the same voltage varies in the storage capacitor Cst, the greatest luminance varies in the green organic light emitting diode OLED(G), whereas the least luminance varies in the blue organic light emitting diode OLED(B). When luminance variation amounts of the red, green, and blue pixels are differently set due to the leakage current, a uniform image cannot be expressed. This causes observers to observe a luminance variation due to the leakage current.